1. Technical Field
The present invention relates in general to monitoring stress in a structure and, in particular, to an improved system, method, and apparatus for a wireless, non-powered stress history and fatigue monitoring of a structure.
2. Description of the Related Art
Many structures, especially in the aerospace industry, must be monitored over time for overload and/or fatigue damage. Typically, this is done in one of two ways. First, a structure may be statistically evaluated for stress or fatigue. With this method the exact number of cycles is not known, but it does offer a prediction of what is typical for a certain amount of running time or flights. Second, active strain gages and/or accelerometers, for example, may be used to count cycles. Unfortunately, the first method is approximate, while the second method is costly and not feasible for most in-service applications due to wiring and power requirements. Some practitioners have proposed “power harvesting” for such applications whereby energy is harvested from vibrations, solar, RF, thermal, or other power sources. Although that solution would eliminate the power wiring requirements or batteries, the proposition is very challenging for continuous operation while a structure is in service.
Examples of externally powered prior art solutions include U.S. Pat. No. 4,433,581 to Scott, which describes strain gages that are powered by conventional means. U.S. Pat. No. 5,520,055 to Fussinger shows a device that has notches that break upon a sufficient number of cycles of fatigue. U.S. Pat. No. 5,531,123 to Henkel uses a fatigue monitor that must be physically removed from the underlying structure and then inspected by viewing striations formed in the monitor due to the fatigue. U.S. Pat. No. 6,014,896 to Schoess discloses a piezoelectric device that emits an acoustic wave and is battery powered. U.S. Pat. No. 6,928,881 to Brennan discloses a power detector with strain gages and a memory device with data storage. Finally, U.S. Patent Application No. 20040078662 to Hamel describes the energy harvesting concept for wireless sensors. Although each of these designs is workable, an improved solution for monitoring the fatigue of components while they are in service and can remain in service after inspection would be desirable.